The capture of CO2 from industrial process streams is gaining increased interest in response to growing concerns about greenhouse gas emissions into the atmosphere. Fossil and biomass-based energy conversion processes convert hydrocarbon materials into carbon dioxide and water while releasing energy. The purpose of CO2 capture is to produce relatively pure CO2 suitable for sequestration (such as geological storage or mineralization) or reuse.
Different process approaches have been devised for CO2 capture in energy conversion processes such as the use of pure oxygen rather than air. In such processes, a flue gas containing only CO2 and water is produced, circumventing dilution with nitrogen.
Alternatively, a separate chemical or physical process is used to extract CO2 from a flue gas obtained via a conventional air-fired combustion process. Known processes for capturing CO2 are for instance scrubbing with monoethanolamine (MEA) or dimethylformamide (DMF), Pressure Swing Adsorption (PSA) or membrane separation.
Other processes make use of the triple point of CO2, which is approximately 5.2 bara and −56.7° C., and the fact that liquid CO2 can only exist at certain temperatures and pressures above the CO2 triple point.
In U.S. Pat. No. 7,073,348 is disclosed a process for the capture of CO2 from flue gas at atmospheric pressure by contacting the flue gas with the external surface of a heat exchanger, while evaporating a refrigerant fluid on the inside. Solid CO2 is deposited on the external walls of the heat exchanger. After a certain operating time, the flow of flue gas on the external part of the exchanger and refrigerant fluid on the inside of the exchanger are respectively switched over to a second parallel heat exchanger. The solid CO2 deposited on the externals surface of the first heat exchanger is reheated from −78.5° C. to −56.5° C. at a pressure of 5.2 bar and the CO2 is retrieved as a liquid phase.
Heat exchangers are expensive and have limited area available for heat exchange and deposition of the solid CO2 As the refrigerant continuously provides cold to the evaporator surface, most of the CO2 will deposit on the upstream side of the evaporator, resulting in an inhomogeneous distribution of the solid CO2. Also, due to the build up of the solid CO2 layer the pressure drop over the evaporator is increased significantly during operation. Furthermore, the resistance to heat transfer increases with the increasing thickness of the deposited solid CO2 layer, resulting in an inefficient use of the refrigerant.
Consequently, it is necessary to operate the expensive and relatively sensitive evaporator apparatus under short deposit/removal cycles thereby exposing the evaporator apparatus to rapid changes in temperature, which is disadvantageous from a mechanical point of view.
U.S. Pat. No. 4,265,088 discloses a process for treating hot exhaust gas using two or more packed towers. In the process of U.S. Pat. No. 4,265,088 the hot exhaust gas is introduced in a packed tower, which was cooled to a temperature below the sublimation temperature of CO2. The CO2 is sublimated and thereby captured from the exhaust gas. The sublimated solid CO2 is subsequently removed from the packed tower by applying a vacuum to the packed tower to induce evaporation of the solid CO2. However, such a process can only be used for treating exhaust gases containing low concentrations of CO2. When the exhaust gas contains high concentration of CO2 the use of a vacuum becomes impractical. Alternatively, U.S. Pat. No. 4,265,088 discloses the use of treated exhaust gas to remove the solid CO2 from the packed tower. However, this has the disadvantage that CO2 is reintroduced in at least part of the treated exhaust gas. As a consequence the CO2 cannot be directly provided to a CO2 sequestration process and a new CO2 contaminated gas stream is formed.